Radiotelephone service has been in use for some time and has traditionally been characterized by a central site transmitting with high power to a limited number of mobile units in a large geographic area. Mobile transmissions, due to their lower power, are received by a network of receivers located remotely from the central site and then returned to the central site for processing.
In yet another configuration, the radio coverage area is divided into contiguous smaller zones of coverage (cells) using lower power transmitters and receivers at the central site. One such system is described in U.S. Pat. No. 3,906,166 assigned to the assignee of the present invention. The limited coverage area enables the channel frequencies used in one cell to be reused in another cell geographically separated according to specified plans. One such plan is disclosed in U.S. Pat. No. 4,128,740, assigned to the assignee of the present invention. Consequently, a large number of channels can be made available in a metropolitan area, and the service appears to be identical to the standard telephone.
Each cell dedicates one or more communication channels as a Common Control Channel (CCCH). The CCCH is made up of several logical channels which provide different services to the mobile subscriber. The Random Access Channel (RACH) is used for receiving channel requests from mobile subscriber units. The Access Grant Channel (AGCH) is used to instruct mobile subscriber units, via channel assignment signals, to tune to a particular frequency and timeslot where further signalling may take place. The CCCH shares the same frequency and timeslot as the Broadcast Control Channel (BCCH) which is used to transmit cell specific parameters to the mobile subscribers tuned to the cell. A typical channel allocation scenario is summarized as follows. A mobile unit sends a channel request over the RACH, starts a guard timer, and awaits a channel assignment signal or an assignment reject signal from the Base Station System (BSS), informing the mobile whether channel allocation was successful or not. Upon expiration of the guard timer, however, the mobile unit will transmit another channel request. This procedure will continue until the mobile unit is successfully assigned a Dedicated Control Channel (DCCH), or until a maximum number of retries has been attempted, as defined by the BSS broadcast channel.
For security purposes, mobile unit channel requests do not contain mobile identification information. Instead, they contain a random reference number selected and remembered by the requesting mobile unit. The BSS therefore has no mobile specific information with which to distinguish one channel request from another. Accordingly, the above described scenario is extremely susceptible to the misallocation of system resources.
If the BSS fails to respond to a mobile unit's initial channel request within the mobile unit's guard time, hereinafter referred to as the channel request retry interval, or simply the retry interval, the mobile unit will send a new request. Since the BSS has no mobile specific information regarding the origin of this request, there is no means to correlate the mobile unit's new request with a request already in progress for this mobile unit. This may result in the BSS processing both requests, thereby activating two channels for the same mobile unit. Since the mobile will respond to the first assignment signal received from the BSS, the second will go unanswered thereby resulting in a wasted channel.
The occasional channel misallocation is not catastrophic to system performance, since the unused channel will eventually be returned to service. However, the increased occurrence of misallocation, or misallocation during periods of heavy use is a formidable obstacle to the efficient operation of a modern cellular radiotelephone communication system. For example, during periods of heavy use, a BSS typically takes more time to process each incoming channel request. Awaiting mobile units, therefore, are more likely to retry channel requests because of the slowed BSS response. As no mobile identity is included in these requests, the BSS will likely respond to both, and activate two channels for a single mobile unit, thereby wasting system resources at a time when resources are already at a premium.
It would be extremely advantageous therefore to provide a method and an apparatus for reducing superfluous channel allocations in a cellular radiotelephone communication system servicing a plurality of mobile units which make multiple requests, thereby reducing the likelihood that more than one channel will be assigned to a single mobile unit.